The present invention is directed to a method and a device for operating a vacuum reservoir provided in an internal combustion engine, in particular of a motor vehicle. The vacuum reservoir supplies the auxiliary power, in the form of a vacuum pressure, needed for at least one servo power-assist (one servo booster) unit, and is acted upon by a vacuum pressure prevailing in an intake manifold of the internal combustion engine and in an, in particular, electrical suction pump.
German Patent No. 31 25 923, and German Published Application No. 44 44 013 discuss power brake and power steering systems in motor vehicles that draw their auxiliary power from a vacuum reservoir which is coupled to an intake manifold. This intake manifold may be used to supply the internal combustion engine with the air (i.e., the oxygen) that is needed for the combustion. In this context, the intake manifold""s vacuum pressure may be stored temporarily in the vacuum reservoir, which may be coupled via a non-return valve to the intake manifold.
Accordingly, to provide adequate servo power assistance, for example servo braking power, a vacuum pressure must have been present for a long enough time in the intake manifold to ensure the proper vacuum pressure in the vacuum reservoir. In response to low manifold pressure, air flows out of the reservoir into the intake manifold. The thus minimally attainable pressure in the vacuum reservoir corresponds to the prevailing manifold pressure. In response to actuation of the brake, the vacuum reservoir is connected via a valve to an actuator which boosts the braking power. In the process, air flows into the vacuum reservoir, thereby increasing the reservoir pressure.
In internal combustion engines of the afore-mentioned type, a throttle valve, which may be used to adjust the air supplied to the combustion chamber, may also be provided in the intake manifold. In conventional internal combustion engines, in particular in the Otto spark-ignition (gas) engine, the throttle valve closes, even when the driver, during braking, removes his/her foot from the accelerator, so that a possibly existing vacuum pressure in the reservoir may be retained. Therefore, in these internal combustion engines, it may be ensured that the vacuum reservoir is able to supply the vacuum pressure required for the servo power-assisted brake even during a relatively long braking action.
However, this is not always ensured in newer internal combustion engines having gasoline direct injection (GDI) or electronic throttle control (e-gas). For example, during the process of heating an existing catalytic converter, the throttle valve may be open to the point where there is no longer an adequate vacuum pressure in the intake manifold. As a result, the vacuum pressure required for the servo system(s) may no longer be able to be made available in the vacuum reservoir.
In addition, in internal combustion engines having GDI or electronic throttle control (e-gas), the throttle valve may be controlled independently of the pedal sensor position, so that the manifold vacuum pressure is restricted in terms of its availability for servo functions.
Examples of this include operating states where manifold injection is accompanied by retarded ignition timing for catalytic converter heating during warm-up. In these operating states, it may be necessary to compensate for a wanted loss of efficiency by opening the throttle valve. This may lead to an increase in the manifold pressure. Stratified operation accompanied by direct injection may be a comparable operating state, where, even at a low load, the throttle valve is fully open, so that, as a result, no manifold vacuum pressure may be available.
In this context, it may also happen during vehicle operation at high altitudes, for example during uphill driving, that the difference relative to the ambient pressure no longer suffices for the servo functions.
The servo power-assisted brake system may be especially critical with respect to safety. If adequate vacuum pressure is not provided, then no brake assistance may be available, or the desired operating state may not be able to run due to safety considerations, thereby leading to deterioration of the exhaust gas or fuel consumption.
A remedy has been discussed in which the throttle opening is designed in such a way that sufficient vacuum pressure is always available. As a result, the throttle opening may not always be able to be optimally designed, in terms of the exhaust gas, for example during the catalyst heating. In addition, in GDI (gasoline direct injection) operated vehicles, a vacuum-operated switch may be used. If the pressure in the brake booster rises above a threshold value, then the switch may be made from stratified operation to homogeneous operation.
In an internal combustion engine, the vehicle may have a built-in suction pump, which compensates for the lacking pressure differential as soon as the pressure in the intake manifold no longer suffices for evacuating the vacuum reservoir. To keep costs low, the suction pump should be a relatively simple component, which is only put into operation in truly necessary cases. Due to cost considerations, the outlay required to detect a necessary switching on or a possible cut off of the suction pump should likewise be kept as low as possible.
It is, therefore, an object of the present invention to provide a method as well as a corresponding device, which, without substantial outlay and with the greatest dependability, will enhance the operational reliability of the vacuum reservoir and minimize mechanical and, thus, also cost expenditures, and at the same time achieving a reliable operation of the vacuum reservoir.
In an example method according to the present invention, quantity flows are fed to the vacuum reservoir, in a computational model, when at least one servo unit is actuated. Quantity flows are removed from the vacuum reservoir when the pressure prevailing in the intake manifold is lower than the pressure prevailing in the vacuum reservoir. Quantity flows are removed from the vacuum reservoir when the electrical suction pump is switched on. The pressure prevailing is determined in the vacuum reservoir from the balance of the flow quantities fed to and removed from the vacuum reservoir.
According to the present invention, the pressure in the vacuum reservoir, and, therefore, also the vacuum pressure in a servo system, for example in the brake booster are determined by calculation. In this context, the pressure differential in the vacuum reservoir, i.e., in the reservoir of the servo system, is calculated by a model. From the balance of air-inflow and air-outflow quantities to and from the reservoir volume, this model determines the specific pressure in the reservoir.
The compressibility of the gas in the reservoir may be considered in a state equation for ideal gases. The flow quantity into the reservoir may be calculated from the difference between the reservoir pressure and the intake manifold pressure and between the reservoir pressure and the minimal suction pump pressure, allowing for the resistances to flow in the lines to the reservoir. Flow quantities flowing out of the reservoir may be determined from the driving states, e.g., cornering, with the power steering system being loaded to this effect, or a vehicle deceleration, with the power brake system being loaded to this effect.
In internal combustion engines, certain output variables may already be known from such a motor control. In anti-lock braking systems (ABS), for example, motor control interfaces are already present which transmit information on the air consumption in the servo brake to the engine management system. Information on intake manifold pressure, ambient pressure, change in vehicle speed, and the operating state of the suction pump is also usually available in conventional engine management systems. The additionally required sensors, such as pressure switches or pressure boost sensors in the pressure reservoir (accumulator), are likewise present in existing systems. Thus, in an implementation of the method according to the present invention, no additional construction measures may be necessary in existing internal combustion engines. In accordance with one embodiment of the present invention, the required differential pressure is compared to the available differential pressure. If there is not sufficient differential pressure available, either the suction pump is turned on or the engine operating state is altered. In the case of a change in the operating state, such a state, accompanied by elevated intake-manifold vacuum pressure, may be adjusted in that the engine control reduces the throttle-valve opening.
In a further refinement of the example embodiment of the present invention, an additional throttling takes place in stratified operation. In the case of catalyst heating using a retarded ignition timing, an ignition timing that is optimal for efficiency is given priority over the catalytic converter heating. As a result, the engine requires less air-mass flow, the throttle valve may be closed, and the intake-manifold pressure decreases.
The changes in the operating state having elevated intake-manifold vacuum pressure may be made furthermore by intervening in the power efficiency specifications or in the operating state specifications of the engine management. In the case of engine management, direct interventions in the ignition timing are also possible without necessitating computational modeling of the efficiency or the operating state.
In engine management concepts for optimizing consumption and exhaust-emission behavior, operating states associated with high intake-manifold pressure occur. To respond to insufficient vacuum pressure in the servo system, the example method provides for either turning on the suction pump and/or altering the operating state to ensure sufficient vacuum pressure in the servo system. In this context, the example method eliminates the need, in particular, for the pressure sensor at the vacuum reservoir or, given the existence of a pressure sensor, eliminates the need for monitoring the sensor for correct functioning.
Another method according to the present invention provides for the quantity flows fed to the vacuum reservoir and/or the quantity flows removed from the vacuum reservoir to be continually summed using the appropriate operational sign, or integrated. The contents of the vacuum reservoir are thus able to be continually and dynamically adapted to the quantity flows existing at any one time and, on the basis of the contents, the pressure prevailing in the reservoir may be predicted at any point in time. In this connection, the quantity flows in question may either be summed as discrete air volumes or be integrated as infinitesimal air-volume variations.
The method according to the present invention may additionally provide that flow quantities be removed from the vacuum reservoir only when the difference between the pressure prevailing in the intake manifold or suction pump and the pressure prevailing in the vacuum reservoir exceeds a predefined threshold value. A non-return valve may be installed between the vacuum reservoir and the intake manifold. This may likewise be considered in conjunction with the proposed threshold value in the model calculation according to the present invention. In this context, the threshold value may be adapted to the particular physical conditions and, thus, may be used as an additional parameter for optimizing the proposed model. Moreover, pressure losses which may-occur due to flow resistance in the line may be considered on the basis of the parameter.
The model calculation in accordance with the present invention may also provide for calculating the quantity flows fed to the vacuum reservoir and/or the quantity flows removed from the vacuum reservoir from operating states of the internal combustion engine, in particular from driving states of the motor vehicle. On the one hand, the pressure prevailing in the intake manifold may depend on the operating states of the internal combustion engine, in so far as the air supplied by the intake manifold to the combustion chamber may depend on the current state of combustion. On the other hand, the air supplied to the intake manifold, for example via the throttle valve, may be controllable. In the case of a motor vehicle, parameters such as vehicular speed or braking deceleration may be consulted. From these, inferences may then be made, in particular, with respect to the air masses supplied via a brake booster to the vacuum reservoir.
In accordance with another embodiment of the method of the present invention, the quantity flows fed to the vacuum reservoir may be calculated using a step function that is triggered by a signal edge representing the servo function response. This measure may be consistent with technical considerations. In the case of a brake booster, for example, when a corresponding braking force is applied to the brake booster in response to a braking maneuver, the quantity flows fed to the vacuum reservoir correspond approximately to a step function having a more or less constant time duration per braking intervention. In this context, the step function may be modeled, in particular, in the form of a monoflop.
Based on the modeling of the pressure in the vacuum reservoir in accordance with the present invention, as a function of the pressure prevailing in the vacuum reservoir, and as calculated from the balance of the flow quantities, an intervention may be made in the internal combustion engine to lower the pressure in the vacuum reservoir. In particular, a throttle valve may be provided in the intake manifold which may be adjusted, and/or the suction pump may be turned on. As a result of this intervention, therefore, the vacuum pressure required for the servo system is again automatically made available in response to the exceeding of a pressure threshold, without the need for any intervention or interaction on the part of the driver. Thus, according to this embodiment, an appropriate vacuum pressure may be assured at all times.
Alternatively or additionally thereto, as a function of the pressure prevailing in the vacuum reservoir, and as calculated from the balance of flow quantities, an appropriate flag (indicator) may be set. In particular, a control or warning signal may be output. Thus, in this exemplary embodiment, an intervention in the internal combustion engine is not necessarily automatically made to lower the pressure in the vacuum reservoir. Rather, an appropriate bit is initially set which is subsequently able to trip a corresponding control or warning signal. This enables the driver to initiate the necessary countermeasures on his/her own accord, or he/she is at least informed about the initiated measures.
In accordance with another exemplary embodiment of the method according to the present invention, the calculated pressure prevailing in the vacuum reservoir may be used for monitoring a pressure sensor for correct functioning. In this connection, the model-based calculation of the pressure prevailing in the vacuum reservoir may be used as a further safety feature for an internal combustion engine of the species and, thus, makes it possible to detect a malfunction of an already existing pressure sensor.
It is emphasized here that, of the various servo functions provided in a motor vehiclexe2x80x94as far as maximum driving safety is concernedxe2x80x94the brake servo (booster) function may be given precedence over the other servo functions. Thus, in accordance with one further refinement of the inventive idea, in the case of insufficient vacuum pressure in the vacuum reservoir, functions such as servo (power) steering or resonance throttle control may be temporarily deactivated, in order to at least ensure an adequate brake servo function. This may be important, for example, when the user demands braking force to such a degree (for example by pumping the brake pedal) that even the electrical suction pump is not able to evacuate quickly enough the air volume from the vacuum reservoir required for this.
The assumption may also be made in a first approximation, when modeling the mass flows flowing into and out of the vacuum reservoir, that the sum of the inflowing and outflowing air flows is constant. Furthermore, in choosing the dimensions of the integrator/summator of the inflowing and outflowing mass flows, as a lower limit one may assume the pressure prevailing in the intake manifold or in the suction pump, and as an upper limit one may assume the ambient pressure.
The present invention is also directed to a computer program which is suited for implementing the above-described method when it is carried out on a computer. In this context, the computer program may be stored in a memory, in particular in a flash memory.
The control unit provided in accordance with the present invention for an internal combustion engine may include means for detecting the actuation of at least one servo function, means for sensing the pressure prevailing in the intake manifold, means for sensing the operating state of the suction pump, and means for calculating the pressure prevailing in the vacuum reservoir. The control unit may provide for quantity flows to be fed to the vacuum reservoir when at least one servo function responds, for quantity flows to be removed from the vacuum reservoir when the pressure prevailing in the intake manifold is less than the pressure prevailing in the vacuum reservoir, and for the pressure prevailing in the vacuum reservoir to be calculated from the balance of flow quantities fed to and removed from the vacuum reservoir. Data-acquisition means are provided and may include a switch or a sensor for detecting the actuation of at least one servo function, a stop-light or brake-light switch for detecting a brake actuation, means for sensing the intake manifold pressure required for the model calculation, and means for sensing the operating state of the suction pump. Means are provided, moreover, for processing this acquired information on the basis of the proposed model.